1. The Field of the Invention
Exemplary embodiments of the invention generally concern a transmission that is capable of defining, and operating over, a large range of gear ratios.
2. Related Technology
From nearly the beginning of mechanical engines, the purpose and design of an engine has been focused, to at least some degree, on allowing a small engine to move a large load. As engines evolved and technology became more sophisticated, engines were developed having transmissions with multiple ratios to allow the engine to start moving the load with a low ratio and to incrementally step up to higher ratios as the load began moving. In this manner, a transmission can make more effective use of the engine's torque and keep the engine operating near an appropriate speed. Moreover, an engine can operate within a narrow range of speeds while providing a wider range of output speeds.
To effect an incremental change in gear ratio, a manual transmission uses various separate driven gears of different sizes in connection with one or more drive gears. As a gear ratio change is made, a drive gear disengages from the driven gear and re-engages with a different gear. For example, a clutch may disengage a drive gear from a driven gear and then re-engage the same or a different drive gear with a second driven gear having a different radius. As the newly engaged gears have different radii—or levers—the gear ratio is changed. To effect this gear ratio change, however, the drive gear must be temporarily disconnected from all driven gears, such that the power source is also temporarily disconnected from the load while the gear ratio change is made. While temporary, the disconnection between the drive and driven gears generally lasts long enough to be perceived by an operator of machinery utilizing the transmission, and long enough that when the drive and driven gears are reconnected, a potentially damaging torque spike may occur.
Automatic transmissions also make incremental changes in gear ratio by disconnecting the engine from the load. To do so, automatic transmissions typically use one or more planetary gear sets which are used in connection with a series of clutches and bands that are controlled by a hydraulic system. To change between gear ratios, valves within the hydraulic system are used to control hydraulic pressure which activates various clutches and bands so as to connect and disconnect the carriers and various gears of the automatic transmission from the engine. Based on the specific clutches and bands that engage and disengage, the transmission achieves a predetermined gear ratio change.
When the power source is disconnected or disengaged from the load, the load must coast until the power source is reconnected. For anything more than disconnection over a negligible amount of time, the load then coasts and significant momentum can be lost. For instance, a semi-tractor trailer or other moving vehicle may be moving uphill when a gear change is required. By pushing in the clutch or otherwise disconnecting the power source of the semi-tractor trailer, the engine RPMs are decreased, turbos may be dumped, and torque can no longer be applied in the movement of the load. As a result, the driver often must shift two or three gears down because re-engaging the power source will not occur fast enough to maintain the engine RPMs at a drop of only one or two gears down. This results in an inefficient use of the engine horsepower and fuel.
Similarly, where a tractor is pulling a load such as a plow, temporarily disconnecting the engine from the load so as to change gear ratio reduces the momentum of the tractor and the plow. While the tractor may be able to coast, the plow is less likely to coast. For example, when the plow loses enough momentum, it may catch on the ground being plowed and thereby drag against and stop the tractor from coasting. The plow may catch and stop with a sudden movement that can damage the tractor and potentially injure the operator. Therefore, to avoid damage and injury, the tractor operator may drive the tractor and plow in a low gear to avoid the need to shift gears although a higher gear would allow the tractor to more quickly plow the field, consume fuel more efficiently, and make use of the momentum to obtain a draft of the plow.
In addition, many other applications have previously been unable to take advantage of the benefits of a variable speed transmission because disconnection of the power source from the load makes the application unsafe or impractical. For example, an elevator could benefit from gear ratio changes to change the speed of its ascent or descent. However, disconnecting the power source during ascent or descent would cause the elevator carriage to coast, or free-fall, and could make the variable speed transmission unsafe for the elevator passengers.
A conveyor system such as those used in manufacturing or mining operations could also benefit from variable speeds. For example, as the system starts up the conveyor belt could be started at a slow speed and the speed then increased for full operation. Many conveyor belts are, however, loaded with material and/or are miles long, thereby creating a large load that must be moved. If a gear ratio change were to be made by even temporarily disconnecting the power source, the material and conveyor belt would lose momentum and prevent an effective gear ratio change. Consequently, materials often have to be removed from the belt just to get the conveyor moving, and/or the conveyor system must be operated at a constant speed.
While variable speed transmissions provide many benefits, the significant disconnection of the power source from the load in these traditional transmissions has caused engine and transmission designers to search for methods and systems that minimize the time the power source is disconnected and a drive gear is disengaged. To at least some degree, automatic engines have reduced this time by automating the shifting between gears and changing gear ratios, thereby also reducing the time between disconnection and reconnection of the power supply to the load. However, even automatic engines disconnect the engine from the drive gears for a time long enough to cause a potentially significant loss in torque, thereby failing to make an efficient use of the available horsepower. Moreover, by operating with only a very limited number of discrete gear ratios, that may be relatively widely spaced, the engine operates mostly in an inefficient range. Consequently, the engine must be capable of providing more horsepower, and must thus be larger, than would otherwise be required if an engine was more frequently running at an efficient speed. The inefficient use of these engines, in turn, burns more fuel than would an engine run at more efficient speeds.
While decreasing the time needed to change between gear ratios also decreases the time during which the load and the power source are disconnected, it can also create greater torque spikes which may damage the drive train. In particular, as a gear ratio change is made from one discrete gear ratio to another discrete gear ratio, engagement of the drive and driven gear may produce a torque spike such that as the drive and driven gears engage, the torque produced momentarily spikes. The torque spike can be reduced by feathering the clutch so as to cause the drive and driven gears to gradually re-engage. If, however, the shift is made too quickly, the torque spike can produce an output torque large enough to damage a drive shaft, chassis, or an axle.
Accordingly, some efforts have been made to reduce a torque spike so as to reduce the likelihood that the torque spike will cause damage. For example, a torque spike anticipator may be used to artificially lower the torque as a gear ratio change is made. In particular, as a gear ratio change is made, the torque spike anticipator may lower the engine RPMs during the gear ratio change, such that as the gears re-engage to produce the new gear ratio, less torque is produced during the torque spike. Such a system adds, however, additional complexities to a transmission and prevents operation at a constant velocity so as to make an efficient use of the available power.
In low torque applications, the problems associated with disconnecting the power source from the load have been reduced, to some extent, by continuously variable transmissions (CVT) and infinitely variable transmissions (IVT). A CVT typically uses two pulleys which are connected by a belt. The pulleys can include two cones that face each other and which can be pulled together or pushed further apart by hydraulic pressure, centrifugal force or spring tension. As one pulley increases its radius, the other decreases its radius to keep the belt tight. As the two pulleys change their radii relative to one another, they create various gear ratios. A similar concept is embodied in an infinitely variable transmission (IVT) which also makes use of similar, complementary pulleys and cones. Instead of a belt, however, the IVT uses a rolling member that is sandwiched between the cones.
Regardless of whether a CVT (wrapping member) or IVT (rolling member) is used, however, the system relies on friction to adjust gear ratios and provide power output. Friction introduces heat into the system, however, and, as a result, the wrapping member and rolling members heat up and are susceptible to wear damage, thereby requiring that the user repair or replace the parts. To reduce the frequency of repair, the frictional wrapping or rolling members can be toughened, such as through the use of a thicker belt or impregnation of the belt with metals or other tougher materials. However, as the belt strength is increased, the part costs increase. Moreover, sufficiently tough materials can cause the cones within the transmission to wear and fail.
Moreover, because these systems are friction-based, they are typically only suitable for low torque applications, as high torque applications could cause excessive heating within the transmission, thereby causing even greater wear and failure of the transmission components. As a result, CVT and IVT transmissions are not scalable for a wide variety of low and high torque applications. In fact, the application of torque to a CVT or IVT in a high torque or high horsepower system may cause near immediate failure as the rolling member or wrapping member can melt or otherwise deteriorate due to the friction-induced heat.
Because the CVT and IVT have been seen as unacceptable alternatives in high-torque applications, additional efforts have been made within high-torque applications so as to provide little to no time gap between disconnection and reconnection of the power source and load. For example, John Deere produces tractors with a PowerShift transmission that uses a complex design which is purported to automatically do the clutching and disconnect a load and reconnect the load at about the same time such that there is no real time gap and little to no torque loss. The transmission is, however, much larger than a standard transmission, and can house a large number of hydraulic lines inside the transmission. As a result, maintenance of the lines may be difficult, and the size of the engine further increases the size of the equipment and the weight or load that must be carried. Moreover, because of the complexity and size of the transmission, it can be cost prohibitive for certain applications, and it is not scalable for low torque or smaller applications.
Accordingly, a need exists for an improved transmission which is scalable and which can switch between any of various gear ratios without requiring disconnection of the power source from the load.